A critical feature of rigid disk magnetic storage devices is the vulnerability to failure by the slider wearing into the magnetic layer on the disk surface. Magnetic performance improvements have been achieved by using thinner magnetic coatings (less than 100 nanometers thick), and lower flying heights (less than 10 microinches). Both of these factors mean that the tribology of the system must be excellent if a useful life is to be achieved. A thin film of lubricant molecules is required as part of the tribological system to keep the coefficient of friction low when the slider lands on the disk, or intermittently hits it while flying.
In the case of earlier magnetic disk data storage devices, the magnetic medium was a magnetic ink that had significant thickness and porosity. A relatively large amount of lubricant could be accommodated by such a disk, so it was not as sensitive to monolayer quantities of adsorbed contaminants as present disks are. Present disks have an overcoat that is only 20 to 50 nanometers thick, and has very little porosity for storing lubricant. In fact, the disk lubricant is typically only one to several monomolecular layers thick. For a given disk design the lubricant thickness must be held to very close tolerances. If the lubricant gets too thin, the coefficient of friction goes up and wear-out occurs sooner. If the lubricant is too thick, the slider will become stuck to the disk in a process called stiction which can be strong enough to prevent the motor from starting up.
Several lubrication strategies are in use today. A lubricant film may be chemically bonded to the disk surface, and a mobile lubricant film may or may not be added on top of it. A mobile film may be used alone through a one-time application of lubricant at time of manufacture. Or, as taught in U.S. Pat. No. 4,789,913 an equilibrium film thickness is maintained on the disk surface by replenishment through the vapor phase from a reservoir of lubricant within the device enclosure.
Once a strategy has been selected for maintaining the correct thickness, it then becomes important to maintain the correct composition of the film. Contaminant molecules in the vapor phase will become incorporated into the lubricant film. It is unlikely that these compounds will improve the lubrication process, and depending on their chemical structure, they may even destroy it. This has occurred on a number of occasions, resulting in the elimination of certain types of chemicals from the components that go into the device because they cause either wear-out or stiction even when present in only trace amounts in the lubricant film.
The invention disclosed herein is designed to control this problem of lubricant film contamination from the vapor phase.
The key variable to be controlled for each molecular species present in the atmosphere of the enclosure is its relative vapor density. The relative vapor density of any given compound at a given temperature is defined as the ratio of the mass of the compound present per unit volume of air to the mass of the compound that is present in a unit volume of air that is saturated with the compound at that temperature. This is analogous to the special case of water for Which this variable is called relative humidity. It is important because the extent to which a molecular species infiltrates the lubricant film is a function of its relative vapor density at the disk surface.
Typically, disk enclosures today are made to be substantially sealed, so the rate at which molecules evaporate from the components such as greases and plastics is greater than the rate at which they leak out of the enclosure. Therefore many of these compounds can be expected to have high relative vapor densities at the disks. In other words, the air is nearly saturated with them.
In the case of U.S. Pat. No. 4,789,913 the relative vapor density of the lubricant in the atmosphere is controlled by the temperature difference between the lubricant reservoir and the disk surfaces. The relative vapor density is deliberately maintained at 0.5 to 0.8 at the disks by the fact that the reservoir is positioned at a location that is 1 to 5 degrees Celsius cooler than the disks during operation. If the temperature difference is allowed to become too small, then the relative vapor density of the lubricant at the disks will get too close to one and the lubricant film will get too thick. If the reservoir gets too cold relative to the disks, then the lube film will get too thin. The spinning of the disks moves the air through the reservoir structure. The reservoir is designed to ensure that the air leaving it is saturated (relative vapor density=1) at the reservoir temperature.
A typical file contains many parts that inadvertently act as reservoirs. Plasticizers from plastic parts, volatile components from greases, and contaminants such as fingerprints, are major sources. Many of these are in locations that are as warm as the disks, so depending on the rate at which they outgas, and the efficiency with which the airflow carries the molecules to the disks, a high relative vapor density may be established at the disks. This will lead to increased contamination of the lubricant film.
There have been recent proposals for even higher density memories designated SXM and based on a STM (Scanning Tunneling Microscope) or other techniques with a head moving extremely close to an extremely smooth memory surface. These offer data density approaching atomic density. These SXM memories will have even greater vulnerability to vapors causing adhesion. Also contamination vapors will cause surface contamination which will obscure atoms of data. Thus these new memories will require even more control of adhesion and vapors compared to thin film magnetic disk memories.